The present invention relates generally to detection of and response to potentially hazardous conditions in an electric vehicle battery pack, and more particularly but not exclusively, to detecting and responding to potentially hazardous over-current due to internal short circuit to limit possible excessive thermal conditions of the individual battery cells and modules.
Many high-performance energy storage solutions now employ series-connected modules that, in turn, are series and parallel combinations of individual battery cells. Battery packs used with electric vehicles store large amounts of energy in a small space, producing high energy densities. The battery packs are designed to provide high levels of safety and stability, yet situations can arise where a portion of a battery pack experiences a local thermal condition which generates significant heat. When the temperature is great enough and sustained, the local thermal condition can transform into a runaway thermal condition affecting wide areas of the battery pack, and sometimes the entire battery pack under certain circumstances. This runaway thermal condition can begin when the local thermal condition approaches the runaway thermal point, which can be about 200° C.
Great care and precise attention to detail is used when assembling and testing battery packs. It is sometimes the case that there will be internal shorts in the battery pack, for example between various of the energy storing sub-units themselves or the energy storing sub-units and other structure (housing, chassis, or the like). These internal shorts can develop during operation for any number of reasons. Prior art battery packs contain fusing architectures designed to both actively and passively interrupt external or internal short circuits above normal operating currents. Most short circuit protection devices have a very short thermal time constant and are only effective above a predetermined current. However, an electric vehicle may only be capable of sustaining operation at its maximum current for a short period of time before internal components, including the battery cells, heat to near their temperature limits. Certain classes of internal short circuits may lead to sustained high current situations below the system's maximum operating current (or below the short circuit protection devices' operation point) but at a sufficiently high current such that a sustained internal short may lead to component over-temperature, failure, and a hazard. Fuses with a long thermal time constant intended to protect the battery pack in this region are expensive and heavy and prior art packs have not implemented an inexpensive or effective method for detecting and responding to such an internal short scenario.
What is needed is an apparatus and method for to detecting and responding to potentially hazardous over-current due to internal short circuit to limit possible excessive thermal conditions of the individual battery cells and modules.